1. Field of the Invention
The present invention generally relates to directional control valves of the type in which the load actuating pressure of a fluid motor is sensed by the directional control valve when the directional control valve is supplying pressurized fluid from a pressure inlet channel to a work port channel.
The present invention also generally relates to load responsive hydraulic systems of the type in which the pressure and the effective output of the pump are controlled to maintain the system pressure at a predetermined pressure magnitude above the highest load actuating pressure that exists in a plurality of directional control valves. The control of the pressure and effective output of a variable displacement pump is achieved by the control of the pump displacement; and the control of the pressure and the effective output of a fixed displacement pump is achieved by a by-pass valve that discharges excess pump flow to a sump.
The present invention more particularly relates to load responsive hydraulic systems of the type in which a flow of signal fluid is supplied from the pump and this flow of signal fluid is used to generate a synthetic signal pressure that is higher than the highest load actuating pressure that exists in a plurality of directional control valves.
The present invention specifically relates to load responsive hydraulic control valves in which each directional control valve furnishes a flow of signal fluid from the pressure inlet channel thereof to the work port channel thereof, and in which a synthetic signal pressure is generated in this flow path from the pressure inlet channel to the work port channel.
The present invention also specifically relates to load responsive hydraulic systems in which a plurality of directional control valves, each providing their own supply of signal fluid only when needed, are interconnected to form a load responsive hydraulic system in which the maximum flow capacity of each directional control valve is increased by the control of the pressure and effective output of the pump by the highest synthetic signal pressure, and in which all shock pressure problems relating to the flow of signal fluids are eliminated by the signal fluid being furnished to each directional control valve only when needed.
2. Description of the Prior Art
The use of a pump supplied fluid for the generation of synthetic signal pressure in load responsive hydraulic systems was disclosed in U.S. Pat. No. 3,971,216 of common inventor entity and common assignee. The synthetic signal pressure that is developed therein is a predetermined value above the highest load actuating pressure of any of the directional control valves that are supplying pressurized fluid to a fluid motor. By controlling the effective output of the pump by this synthetic signal pressure, the pressure differential across a directional control valve, from a pressure inlet channel to the work port channel, is increased and therefore the maximum flow capacity of the directional control valve is increased.
The above referenced load responsive hydraulic system, of the synthetic signal type, achieves the advantage of a low stand-by pressure for the minimization of power loss and heat rise during stand-by conditions and also achieves the additional advantage of a relatively high differential pressure, from a pressure inlet channel to a respective work port channel of any of the directional control valves, for achieving good flow capacity to the work port channels. However, the synthetic signal type of load responsive hydraulic system has the inherent disadvantage of producing pressure surges in the output pressure due to the flow of signal fluid and resultant timing problems in the individual directional control valve.
Schurger, in U.S. Pat. No. 3,878,864, disclosed a load responsive hydraulic system in which a signal fluid was supplied from the pump and a unique by-pass valve to the directional control valve only after the load actuating pressure from one of the control valves was supplied to the special and rather complex by-pass valve. The use of this by-pass valve was effective to start the flow of signal fluid only when needed and thus eliminated shock pressures which ordinarily would occur in a load responsive system of the synthetic signal type when a directional control valve is moved from a stand-by position to an operating position; but it was not effective to stop the flow of fluid before an unnecessarily high shock peak was developed when the directional control valve was moved from the operating position back to a stand-by position.
In this same patent, Schurger disclosed a load responsive control valve which, when used in conjunction with his specially designed by-pass valve, would eliminate the shock peak that ordinarily would be incurred when the valve spool of the directional control valve were moved from an operating position back to a stand-by position.
In U.S. Pat. No. 4,089,169 of common inventor entity and common assignee as that of the present invention, a load responsive hydraulic system is disclosed that includes a logic system that is effective to control the flow of signal fluid to the load responsive directional control valves only after an attenuation flow path in one of the directional control valves is occluded. This unique logic system is effective to solve, with less complexity and lower cost than the Schurger by-pass valve, the shock pressure peaks which are associated with moving a valve spool of the directional control valve from stand-by position to an operating position; and, when used in a load responsive system having directional control valves similar to those that are disclosed by Schurger, is effective to eliminate all shock pressure problems which are associated with load responsive hydraulic systems of the synthetic signal type.
In the FIG. 3 embodiment of U.S. Pat. No. 3,971,216, a directional control valve was disclosed in which the synthetic signal fluid is furnished from either of a pair of pressure inlet channels of a directional control valve to a control port of the directional control valve by a pair of valved flow paths. This FIG. 3 embodiment is similar to the present invention in that a valved signal path was provided; but it differs in that no provision was made to time the opening and the closing of these valved signal paths with the opening and closing of the fluid flow paths between the respective ones of the work port channels and the return channels.